1. Field of the Invention
This invention relates to a molten salt leachant for core materials used in casting and solidifying advanced superalloy materials.
2. Description of the Prior Art
The production of intricate castings of directionally solidified (DS) eutectic alloys requires the use of chemically stable ceramic cores and molds. The currently available, SiO.sub.2 -based core materials do not possess the chemical stability required for casting eutectic alloys. New core materials based on Al.sub.2 O.sub.3, Y.sub.2 O.sub.3, La.sub.2 O.sub.3 and/or MgO have been found to be chemically resistant to eutectic alloys under DS casting conditions. These new core materials are, however, generally inert to the leaching media used for the current SiO.sub.2 -based core materials.
At present silica cores are conventionally removed by leaching in caustic solutions in an autoclave. Several different techniques for core removal other than leaching have been suggested and some have been pursued at various levels. Among these are: techniques based on reactions of the core material to form volatile species such as refractory metal oxides; use of core materials that can be powdered by reactions like hydriding; and even mechanical core removal by techniques such as high pressure water blasts and ultrasonic disintegration. Very little success, however, has been experienced with mechanical core removal techniques on complicated blade configuration.
Chemical core removal techniques, including conventional leaching, present a basic contradiction for core materials selection. The alloy compatibility requirement dictates outstanding chemical stability at high temperatures, while core removal requires high chemical reactivity under relatively mild conditions at low temperatures. This contradiction in chemical stability is the single most restrictive aspect of all the core material requirements. The development of new ceramic cores for eutectics should involve a thorough search for conventionally leachable ceramics; however, other ceramics that have potential for providing the required compatibility must not be excluded. For the latter ceramics other core removal techniques must be developed.
New solvents are, therefore, required which are aggressive toward the ceramic but, at the same time, nondestructive toward the alloy. Molten salts have been found to satisfy the leachant requirements for these new core materials.
It is therefore an object of this invention to provide a new and improved leachant for ceramic cores employed in casting and solidifying advanced superalloy materials.
Another object of this invention is to provide a molten salt bath to leach core materials from advanced superalloy materials without detrimentally affecting the finish of the casting.
A further object of this invention is to provide a new and improved method to remove core materials from castings of advanced superalloy materials which includes a molten salt leachant and a molten salt rinse.
Other objects of this invention will, in part, be obvious and will, in part, appear hereinafter.